#include <iostream>
#include <cstdio>
#include <algorithm>
#include <string.h>
#include <vector>
using namespace std;
#define endl '\n'
#define pi pair<int, int>
const int maxn = 200000;
int n, k;
string s;
int a[maxn], dp[maxn][3];
vector<int> nx[3];
int main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> k;
cin >> s;
int ret = n;
for(int i = 0; i < n; i++){
a[i] = s[i] == 'J' ? 0 : s[i] == 'O' ? 1 : 2;
nx[a[i]].push_back(i);
for(int j = 0; j < 3; j++){
int m = nx[j].size();
dp[i][j] = (m >= k ? j ? dp[nx[j][m - k]][j - 1] : nx[j][m - k] : -n);
}
ret = min(ret, i - dp[i][2]);
}
cout << (ret == n ? -1 : ret - 3 * k + 1) << endl;
return 0;
}
